1. Field of the Invention
This invention relates to a dielectric filter comprising a plurality of dielectric coaxial resonators and a method of adjusting the resonance frequency of the same.
2. Prior Art
There have been proposed various types of dielectric filters, each comprising a plurality of dielectric coaxial resonators juxtaposed in a dielectric ceramic block or substrate in which through holes are formed in the dielectric block in an axial direction, an inner conductive film is provided on the interior wall of each of the through holes for forming an inner conductor, one end of each of the inner conductors is connected to an outer conductive film provided on the outer peripheral surface of the dielectric ceramic block for forming a short-circuit end, and the other end of each inner conductor is separated from the outer conductive film for forming an open-circuit end. These dielectric filters may be in general divided into two groups: one having capacity-coupling input/output terminals as shown in FIG. 1 and the other having magnetic field-coupling input/output terminals as shown in FIG. 2.
In a conventional dielectric filter f1 comprising capacitive-couping input/output terminals e1 which are capacitively coupled to respective outer dielectric coaxial resonators y as shown in FIG. 1, ends of the right and left dielectric coaxial resonators y are made relatively longer than that of the central dielectric coaxial resonator x to adjust the resonance frequency of each dielectric coaxial resonator. That is, as disclosed in Japanese U.M. Kokai No. 60-98902, in this dielectric filter f1, the resonance frequency of each of the dielectric coaxial resonators y disposed on both sides of the dielectric coaxial resonator x is liable to shift to a higher value than that of the dielectric coaxial resonator x. Then, the dielectric coaxial resonators y are extended at ends (lower ends in the figure) to increase the resonance lengths thereof so as to adjust the resonance frequencies thereof.
In an another conventional dielectric filter f2 comprising magnetic field-coupling input/output terminals e2 field coupled to respective outer dielectric coaxial resonators y through conductive through holes, as shown in FIG. 2, one end of the central dielectric coaxial resonator x is made relatively longer than that of right and left dielectric coaxial resonators y to adjust the resonance frequency thereof. That is, with this dielectric filter f2, the resonance frequency of the dielectric coaxial resonators y on both sides of the central dielectric coaxial resonator x are liable to shift to a lower value than that of the central dielectric coaxial resonator x. Then, the resonators y are shortened at lower ends in the figure to adjust the resonance frequency thereof.
The above mentioned dielectric filters f1 and f2 of FIGS. 1 and 2 are of an inter-digital structure in which the directions of the dielectric coaxial resonators are opposite to one another alternately. In such inter-digital type dielectric filters, short-circuit ends appear alternately on one-end side. Therefore, when the short-circuit ends are to be formed, it is necessary to form a conductive layer of a predtermined pattern by means of screen printing or immersion coating or plating after a portion around the open-circuit end is masked by screen printing because such a conductive layer cannot be formed by coating all over the surface or immersion coating on one end side.
However, in the above arrangements that one ends of the outer resonators and one end of the central resonator are extended, uneven surfaces z1 and z2 having a level difference of several millimeters are formed on lower end sides in the figures of the dielectric filters f1 and f2, respectively. Therefore, when a desired pattern is to be formed by thick-film printing or plating with masking, the uneven surfaces make printing difficult and thereby uniform coated surfaces cannot be obtained with the result of a low yield. When screen printing is carried out on these uneven surfaces, the screen may be easily broken by the level differences of the uneven surfaces at the time of printing.
Further, in the case where polishing is carried out to adjust resonance length for a sintered ceramic, to obtain a predetermined degree of input/output coupling, a polishing step becomes complicated because a smooth surface formed near the input/output terminal cannot be polished and the above uneven surface needs to be polished but cannot be ground or polished uniformly. As a result, this causes an increase in the number of steps.
Meanwhile, such arrangement that facilitates the adjustment of resonance length is required not only for the above inter-digital structure but also a comb-shaped structure in which short-circuit ends and open-circuit ends are located on the same sides, respectively.
It is therefore an object of the present invention to solve the above problems and thus to provide a dielectric filter which is capable of equalizing the resonance frequencies of all the dielectric coaxial resonators with ease and a method for adjusting the resonance frequency of such dielectric filter.